Symbols
=======

BFD trys to maintain as much symbol information as it can when it moves information from file to file. BFD passes information to applications though the asymbol structure. When the application requests the symbol table, BFD reads the table in the native form and translates parts of it into the internal format. To maintain more than the infomation passed to applications some targets keep some information 'behind the sceans', in a structure only the particular back end knows about. For example, the coff back end keeps the original symbol table structure as well as the canonical structure when a BFD is read in. On output, the coff back end can reconstruct the output symbol table so that no information is lost, even information unique to coff which BFD doesn't know or understand. If a coff symbol table was read, but was written through an a.out back end, all the coff specific information would be lost. (.. until BFD 2 :).

The symbol table of a BFD is not necessarily read in until a canonicalize request is made. Then the BFD back end fills in a table provided by the application with pointers to the canonical information.

To output symbols, the application provides BFD with a table of pointers to pointers to asymbols. This allows applications like the linker to output a symbol as read, since the 'behind the sceens' information will be still available.

Reading Symbols
---------------

There are two stages to reading a symbol table from a BFD; allocating storage, and the actual reading process. This is an excerpt from an appliction which reads the symbol table:

       unsigned int storage_needed;
       asymbol **symbol_table;
       unsigned int number_of_symbols;
       unsigned int i;

       storage_needed = get_symtab_upper_bound (abfd);

       if (storage_needed == 0) {
          return ;
       }
       symbol_table = (asymbol **) malloc (storage_needed);
         ...
       number_of_symbols =
          bfd_canonicalize_symtab (abfd, symbol_table);

       for (i = 0; i < number_of_symbols; i++) {
          process_symbol (symbol_table[i]);
       }

All storage for the symbols themselves is in an obstack connected to the BFD, and is freed when the BFD is closed.

Writing Symbols
---------------

Writing of a symbol table is automatic when a BFD open for writing is closed. The application attaches a vector of pointers to pointers to symbols to the BFD being written, and fills in the symbol count. The close and cleanup code reads through the table provided and performs all the necessary operations. The outputing code must always be provided with an 'owned' symbol; one which has come from another BFD, or one which has been created using bfd_make_empty_symbol.

An example showing the creation of a symbol table with only one element:

     #include "bfd.h"
     main()
     {
       bfd *abfd;
       asymbol *ptrs[2];
       asymbol *new;

       abfd = bfd_openw("foo","a.out-sunos-big");
       bfd_set_format(abfd, bfd_object);
       new = bfd_make_empty_symbol(abfd);
       new->name = "dummy_symbol";
       new->section = (asection *)0;
       new->flags = BSF_ABSOLUTE | BSF_GLOBAL;
       new->value = 0x12345;

       ptrs[0] = new;
       ptrs[1] = (asymbol *)0;

       bfd_set_symtab(abfd, ptrs, 1);
       bfd_close(abfd);
     }

     ./makesym
     nm foo
     00012345 A dummy_symbol

Many formats cannot represent arbitary symbol information; for instance the a.out object format does not allow an arbitary number of sections. A symbol pointing to a section which is not one of .text, .data or .bss cannot be described.

typedef asymbol
---------------

An asymbol has the form:

     typedef struct symbol_cache_entry
     {

A pointer to the BFD which owns the symbol. This information is necessary so that a back end can work out what additional (invisible to the application writer) information is carried with the symbol.

       struct _bfd *the_bfd;

The text of the symbol. The name is left alone, and not copied - the application may not alter it.

        CONST char *name;

The value of the symbol.

        symvalue value;

Attributes of a symbol:

     #define BSF_NO_FLAGS    0x00

The symbol has local scope; static in C. The value is the offset into the section of the data.

     #define BSF_LOCAL	0x01

The symbol has global scope; initialized data in C. The value is the offset into the section of the data.

     #define BSF_GLOBAL	0x02

Obsolete

     #define BSF_IMPORT	0x04

The symbol has global scope, and is exported. The value is the offset into the section of the data.

     #define BSF_EXPORT	0x08

The symbol is undefined. extern in C. The value has no meaning.

     #define BSF_UNDEFINED	0x10

The symbol is common, initialized to zero; default in C. The value is the size of the object in bytes.

     #define BSF_FORT_COMM	0x20

A normal C symbol would be one of: BSF_LOCAL, BSF_FORT_COMM,  BSF_UNDEFINED or BSF_EXPORT|BSD_GLOBAL

The symbol is a debugging record. The value has an arbitary meaning.

     #define BSF_DEBUGGING	0x40

The symbol has no section attached, any value is the actual value and is not a relative offset to a section.

     #define BSF_ABSOLUTE	0x80

Used by the linker

     #define BSF_KEEP        0x10000
     #define BSF_KEEP_G      0x80000

Unused

     #define BSF_WEAK        0x100000
     #define BSF_CTOR        0x200000
     #define BSF_FAKE        0x400000

The symbol used to be a common symbol, but now it is allocated.

     #define BSF_OLD_COMMON  0x800000

The default value for common data.

     #define BFD_FORT_COMM_DEFAULT_VALUE 0

In some files the type of a symbol sometimes alters its location in an output file - ie in coff a ISFCN symbol which is also C_EXT symbol appears where it was declared and not at the end of a section. This bit is set by the target BFD part to convey this information.

     #define BSF_NOT_AT_END    0x40000

Signal that the symbol is the label of constructor section.

     #define BSF_CONSTRUCTOR   0x1000000

Signal that the symbol is a warning symbol. If the symbol is a warning symbol, then the value field (I know this is tacky) will point to the asymbol which when referenced will cause the warning.

     #define BSF_WARNING       0x2000000

Signal that the symbol is indirect. The value of the symbol is a pointer to an undefined asymbol which contains the name to use instead.

     #define BSF_INDIRECT     0x4000000

       flagword flags;

A pointer to the section to which this symbol is relative, or 0 if the symbol is absolute or undefined. Note that it is not sufficient to set this location to 0 to mark a symbol as absolute - the flag BSF_ABSOLUTE must be set also.

       struct sec *section;

Back end special data. This is being phased out in favour of making this a union.

       PTR udata;
     } asymbol;

Symbol Handling Functions
-------------------------

get_symtab_upper_bound
......................

Returns the number of bytes required in a vector of pointers to asymbols for all the symbols in the supplied BFD, including a terminal NULL pointer. If there are no symbols in the BFD, then 0 is returned.

     #define get_symtab_upper_bound(abfd) \
          BFD_SEND (abfd, _get_symtab_upper_bound, (abfd))

bfd_canonicalize_symtab
.......................

Supplied a BFD and a pointer to an uninitialized vector of pointers. This reads in the symbols from the BFD, and fills in the table with pointers to the symbols, and a trailing NULL. The routine returns the actual number of symbol pointers not including the NULL.

     #define bfd_canonicalize_symtab(abfd, location) \
          BFD_SEND (abfd, _bfd_canonicalize_symtab,\
                       (abfd, location))

bfd_set_symtab
..............

Provided a table of pointers to to symbols and a count, writes to the output BFD the symbols when closed.

     boolean bfd_set_symtab(bfd *, asymbol **, unsigned int );

bfd_print_symbol_vandf
......................

Prints the value and flags of the symbol supplied to the stream file.

     void bfd_print_symbol_vandf(PTR file, asymbol *symbol);

bfd_make_empty_symbol
.....................

This function creates a new asymbol structure for the BFD, and returns a pointer to it.

This routine is necessary, since each back end has private information surrounding the asymbol. Building your own asymbol and pointing to it will not create the private information, and will cause problems later on.

     #define bfd_make_empty_symbol(abfd) \
          BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd))